1. Field of the Invention
The present invention relates to a distortion compensation circuit incorporated in a communication apparatus that is used for satellite communications, ground microwave communications, mobile telecommunications and the like.
2. Description of the Related Art
Conventional linearizers will be described with reference to the drawings. FIG. 9 is a diagram showing the basic structure of a first conventional linearizer disclosed in, e.g., xe2x80x9cThe Institute of Electronics, Information and Communication Engineers, Shingaku-Giho, MW 96-152, 1997.xe2x80x9d
In FIG. 9, reference numeral 1 denotes an input terminal; 2, an output terminal; 31, a bias terminal; 32, a resistor; 33, a diode; and 34 and 35, capacitors.
This linearizer is an example of an analog predistortion linearizer comprising an analog nonlinear element. This analog predistortion linearizer is a linearizer comprising an analog nonlinear element having amplitude and phase characteristics that are opposite to those of an amplifier with respect to input power changes. While connected in series with an amplifier in the preceding or succeeding stage of the amplifier, the linearizer compensates for the nonlinear characteristics of the amplifier, i.e., the amplitude and phase characteristics of the amplifier with respect to input power changes.
A signal is inputted to the input terminal 1, and led to the anode terminal of the diode 33. A bias voltage is applied to the diode 33 from the bias terminal 31 through the resistor 32. The diode 33 provides the effect of compensating for the amplitude and phase characteristics of the amplifier connected to the preceding or succeeding stage of the linearizer with respect to changes in the power level of the inputted signal. The thus compensated signal is outputted from the output terminal 2.
Next, a second conventional linearizer will be described with reference to the drawing. FIG. 11 is a diagram showing the basic structure of the second conventional linearizer disclosed in, e.g., xe2x80x9cIEEE Journal on Selected Areas in Communication, vol. SAC-5, no. 5, pp. 890-895, Apr. 1987.xe2x80x9d
Reference numeral 1 denotes an input terminal; 2, an output terminal; 7, a directional coupler; 11, an amplifier; 25, a local oscillator; 41, a modulated signal generator; 42, an orthogonal modulator; 43, an orthogonal demodulator; and 44 and 45, reverse-phase adders.
This linearizer is a Cartesian feedback linearizer, which is a type of a feedback linearizer. This feedback linearizer extracts part of an output signal from the amplifier, and the extracted signal is negatively fed back to the input side to thereby compensate for the nonlinear characteristics of the amplifier. Of those linearizers, in a transmitter having an orthogonal modulator, one that is called a Cartesian feedback linearizer is characterized as demodulating an output signal from the amplifier into two orthogonal base band frequency components and subjecting the orthogonal components to negative feedback processes, respectively.
The nonlinear characteristics of the amplifier is compensated for by the following way. Transmitted data is inputted to the input terminal 1, and a modulated signal is generated per orthogonal component by the modulated signal generator 41, and the orthogonal modulator 42 modulates the signal into a radio frequency from the base band frequency. The modulated signal is inputted to the amplifier 11, and part of its output signal is extracted by means of the directional coupler 7. The extracted signal is demodulated into two orthogonal base band frequency components by the orthogonal demodulator 43, and the demodulated components are negatively fed back to the input side.
As shown in FIG. 9, the first conventional linearizer comprises an analog nonlinear element, and usually, its circuit is designed and its bias is set so that the linearizer has the amplitude and phase characteristics opposite to those of the amplifier with respect to input power changes. However, it is, actually, impossible to design amplitude and phase characteristics that are completely opposite to those of the amplifier using analog linear and nonlinear elements. Thus, it is so designed, in reality, that a maximum distortion compensation amount can be obtained when the amplifier outputs a specified average power.
FIG. 10 shows a relationship between the distortion compensation amount and the average output power of the amplifier when the first conventional linearizer is used. This linearizer can implement the maximum distortion compensation amount at a specified average output P1, but its distortion compensation amount is reduced at an output P2 that is deviated from the specified average output P1. Thus, when the first conventional linearizer is used, the distortion compensation amount is greatly reduced at power levels other than the designed power level.
Further, as described above, the second conventional linearizer is a Cartesian feedback linearizer, which is a type of a feedback linearizer. As shown in FIG. 11, this linearizer compensates for the nonlinear characteristics of the amplifier by extracting part of an output signal from the amplifier, and negatively feeding the extracted signal back to the input side. However, at the reverse-phase adders 44 and 45, input signals do not coincide with feedback signals from the output side timewise, and thus, when an input signal changes at a high speed timewise, i.e., when the input signal contains a wide band of frequencies, this linearizer cannot keep distortion compensation automatically optimized.
The present invention has been made to overcome the aforementioned problems, and therefore has an object of the invention to provide a distortion compensation circuit capable of always optimizing distortion compensation independently of the average power changes of an input signal, the secular changes of an amplifier and the state changes of the amplifier caused by temperature changes, and capable also of compensating for distortions even when an input signal contains a wide band of frequencies.
A distortion compensation circuit according to the present invention comprises: a vector adjuster for electrically adjusting amplitude and phase characteristics of an input signal; a linearizer, constructed of analog linear and nonlinear elements and having amplitude and phase characteristics opposite to those of an amplifier with respect to changes in input power, which is an output from the vector adjuster, for electrically adjusting the amplitude and phase characteristics with respect to the changes in input power, the amplifier being connected to the succeeding stage of the linearizer; a linear signal extraction path for extracting part of the input signal from the input side of the vector adjuster; a nonlinear signal extraction path for extracting part of an output signal from the output side of the amplifier; a level detector for detecting a combined power level of the signals from the linear signal extraction path and the nonlinear signal extraction path; and a control circuit for adjusting a bias of the linearizer in accordance with the detected combined power level, electrically adjusting the linearizer so that the power detected by the level detector is minimized, and adjusting the vector adjuster so that the power detected by the level detector is minimized every time the linearizer is adjusted.
Further, in the distortion compensation circuit according to the present invention, the linear signal extraction path has a signal path constructed of the vector adjuster, the linearizer and the amplifier, and a delay circuit for making delay characteristics of the nonlinear signal extraction path equivalent to delay characteristics of the linear signal extraction path.
Further, the distortion compensation circuit according to the present invention further comprises: a distortion amplification path for causing an auxiliary amplifier to amplify a combined signal obtained by combining the signal of the linear signal extraction path and the signal of the nonlinear signal extraction path; and an output signal path having a second delay circuit for making delay characteristics of the nonlinear signal extraction path and the distortion amplification path equivalent to delay characteristics thereof to thereby form a feed-forward amplifier for compensating for nonlinear characteristics of the amplifier by combining an output signal from the distortion amplification path and an output signal from the output signal path so as to be anti-phase to each other, wherein the level detector extracts a signal at the input side of the auxiliary amplifier in the distortion amplification path, and detects a power level of the extracted signal.
Further, the distortion compensation circuit according to the present invention further comprises: a distortion amplification path for causing an auxiliary amplifier to amplify a combined signal obtained by combining the signal of the linear signal extraction path and the signal of the nonlinear signal extraction path; and an output signal path having a second delay circuit for making delay characteristics of the nonlinear signal extraction path and the distortion amplification path equivalent to delay characteristics thereof to thereby form a feed-forward amplifier for compensating for nonlinear characteristics of the amplifier by combining an output signal from the distortion amplification path and an output signal from the output signal path so as to be anti-phase to each other, wherein the level detector extracts a signal at the output side of the auxiliary amplifier in the distortion amplification path, and detects a power level of the extracted signal.
Further, the distortion compensation circuit according to the present invention comprises, in place of the linearizer, a reverse distortion amplifier having amplitude and phase characteristics opposite to those of the amplifier connected to the succeeding stage thereof with respect to input power changes, wherein the control circuit electrically controls the reverse distortion amplifier and the vector adjuster in accordance with the detected combined power level.
Further, the distortion compensation circuit according to the present invention further comprises: a first frequency converter, inserted into the succeeding stage of the linearizer, for converting an output signal from the linearizer from an intermediate frequency to a radio frequency; and a second frequency converter, inserted into the nonlinear signal extraction path, for converting an output signal from the amplifier from a radio frequency to an intermediate frequency.